The present invention relates to microelectromechanical system (MEMS) actuators and, in particular, to thermal microelectromechanical system actuators that are activated by Joule heating.
Microelectromechanical system (MEMS) actuators provide control of very small components that are formed on semiconductor substrates by conventional semiconductor (e.g., CMOS) fabrication processes. MEMS systems and actuators are sometimes referred to as micromachined systems-on-a-chip.
One of the conventional MEMS actuators is the electrostatic actuator or comb drive. Commonly, such actuators include two comb structures that each have multiple comb fingers aligned in a plane parallel to a substrate. The fingers of the two comb structures are interdigitated with each other. Potential differences applied to the comb structures establish electrostatic interaction between them, thereby moving the comb structures toward each other.
Advantages of the electrostatic actuator are that they require low current, which results in small actuation energy, and have a relatively high frequency response. Disadvantages are that they require high drive voltages (e.g., tens or hundreds of volts) and large areas and provide low output forces. For example, this type of actuator can produce a force of 0.0059 nN/volt2 per comb-finger height (xcexcm) and can yield a typical actuator force density of about 20 xcexcN/mm2, with the area referring to the surface area of the actuator. Comb drive (electrostatic) actuators used for deployment of microstructures typically occupy many times the area of the device they are deploying. Also, the high voltages (e.g., tens or hundreds of volts) required to operate electrostatic actuators can be incompatible or present difficult integration with conventional logic and low voltage electronics.
A pseudo-bimorph thermal actuator is an alternative to the electrostatic actuator. These actuators utilize differential thermal expansion of two different-sized polysilicon arms to produce a pseudo-bimorph that deflects in an arc parallel to the substrate. Such a thermal actuator produces much higher forces (100-400 times) than comb drive actuators of equal size and can operate on very low voltages and can achieve about 450 xcexcN per/mm2 of MEMS chip area. A disadvantage is the additional electrical power that is required. Two or more actuators may be coupled to a common beam through bending yokes to produce a near-linear movement, which is usually desired in MEMS systems. However, the bending of such yokes consumes much of the force output of the actuators.
The present invention includes a unilateral in-plane thermal buckle-beam microelectrical mechanical actuator formed on a planar substrate of semiconductor material, for example. The actuator includes first and second anchors secured to the substrate along one side of an elongated floating in-plane shuttle that is movable relative to the substrate. First and second sets of elongated thermal half-beams are secured between the floating in-plane shuttle and the respective first and second anchors.
An elongated cold beam is aligned transverse to the elongated floating in-plane shuttle and has one end coupled thereto and the other end coupled to the substrate through the insulating nitride layer. The half-beams are formed of semiconductor material, such as polysilicon. A current source directs electrical current through the thermal half-beams via the anchors to cause heating and thermal expansion of the thermal half-beams. With the motion constraint imparted by the cold beam, the thermal expansion imparts near-linear motion of the floating in-plane shuttle generally parallel its length and generally parallel to the substrate. In one implementation, the half-beams are configured at a bias angle to give the floating shuttle an affinity for in-plane motion.
The resistivity of polysilicon allows the actuator to operate at voltages and currents compatible with standard integrated circuitry (e.g., CMOS). In addition, actuators according to the present invention are very small in area, have relatively high force, and can provide relatively long actuation displacements (e.g. 10-20 microns) at very small increments, making them suitable for deployment of MEMS devices as well as providing minute adjustments in MEMS systems. In one implementation, the present actuator array can produce a force of about 3700 xcexcN per square mm and with 1.53 mW per xcexcN of power. This electrically stimulated movement can be used in micro-motors, optical scanning devices, MEMS deployment mechanisms and other areas requiring mechanical movement on a micro scale.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.